Water vapour is a key component of the climate system. It is the most potent greenhouse gas, and its condensed forms (liquid and ice) exert a profound influence on both incoming solar and outgoing infrared radiation. Climate models find that predictions of climate change are very sensitive to water vapour and cloud feedback: water vapour feedback alone doubles the effect of an increase in other greenhouse gases. But there is considerable uncertainty in these calculations: the water vapour feedback occurs mainly in the upper part of the troposphere where there is no physical link between water vapour and temperature. Climate models tend to suggest that the relative humidity of the upper troposphere will remain unchanged in a warmer world (so that the absolute humidity will increase) but since they do not represent explicitly the processes whereby water vapour gets to those altitudes this result is uncertain. A better understanding of the distribution of atmospheric humidity, and the processes that control it above the boundary layer, is therefore of primary importance in climate research. Water vapor is also, by its presence or its absence, an important component of atmospheric chemistry. The fact that the stratosphere is very dry, with only 4 to 6 water molecules per million, is essential in the ozone chemistry. Although modelling of the future evolution of the stratosphere is still in its infancy, variations of vapour water are likely to be a determining factor and the available records already display signs of a recent evolution. The entry of water vapour in the stratosphere is governed by processes occuring in the region of the tropical tropopause which are strongly coupled with tropical convction. The goal of the school is to present a comprehensive overview of the state_of-the-art and recent progresses in the observation, assimilation and understanding of the distribution of atmospheric water vapour and its role in the climate system.